5 Results and discussion

5.1 The maps
Maps of estimated roadside CO concentrations for 1998 and 2004 are shown in Figures 6 to 14 .
Maps for both typical and extreme meteorology are included for 1998 and maps for extreme meteorology only are included for 2004. The maps show maximum 8-hour concentrations for urban major roads for the UK and for London, Birmingham and Manchester in more detail. Built up road links in Great Britain have been defined as urban. Road links in Northern Ireland have been defined as urban if the land cover is more than 50% urban/suburban in any 1 km x 1 km square intersected by the road link.

Figure 6       Major urban roads excluding motorways 1998 - typical meteorology
Figure 7       Major urban roads excluding motorways 1998 -extreme meteorology
Figure 8       Major urban roads excluding motorways 1998 -extreme meteorology
Figure 9       London - Major urban roads excluding motorways 1998 - typical meteorology
Figure 10      London - Major urban roads excluding motorways 1998 - extreme meteorology
Figure 11      London - Major urban roads excluding motorways 2004 - extreme meteorology
Figure 12      Birmingham -Major urban roads excluding motorways 1998 - typical meteorology
Figure 13      Birmingham -Major urban roads excluding motorways 1998 - extreme meteorology
Figure 14      Birmingham -Major urban roads excluding motorways 2004 - extreme meteorology

Table 4 summarises the numbers of urban major road links expected to exceed the proposed limit value in 1998 and 2004 and the NAQS objective in 1998 and 2003. The road links have been classified into six groups: Inner London, Outer London, rest of England, Scotland, Wales and Northern Ireland.

Roadside CO concentrations in Northern Ireland have been estimated on the same basis as those in the rest of the UK. In contrast to the projections for background sites in Northern Ireland included in Table 2, it has been assumed that maximum CO concentrations at roadside locations are likely to be dominated by road traffic emissions.

Table 4. Summary of mapping results (maximum running 8-hour CO concentrations, ppm)

No roads are expected to exceed the proposed limit value of 8.59 ppm in 2004 for typical meteorology and one road link is estimated to be at risk of exceeding this concentration for extreme meteorology, with a concentration of 9 ppm. This link is Park Lane (A4202) in Inner London and since this road has an open aspect on one side, it is likely that the concentration has been overestimated.

The road links with the highest concentrations are listed in Table 5. The model of roadside CO concentration that we have adopted has the same background CO contribution throughout the country. The distribution of the road links with the highest concentrations is therefore more uniform than for pollutants such as annual mean NO₂ or PM₁₀ for which the contribution of background concentrations varies considerably, with the highest contribution in Inner London.

Table 5. The road links with the highest estimated roadside maximum 8-hour CO concentrations (typical meteorology, ppm)

Table 6 shows the total number of urban major road links with concentrations greater than 8.59 and 10 ppm for each year between 1998 and 2004. No road links are expected to have concentrations higher than 8.59 ppm by 2004 and only 2 are expected to have concentrations greater than the proposed limit value by 2002 for typical meteorology (these two road links are A4020 Park Lane and A406 North circular Road, as listed in Table 5). One road link is expected to have a concentration higher than 8.59 ppm by 2004 and only 14 are expected to have concentrations greater than the proposed limit value by 2002 for extreme meteorology. These 14 links are listed in Table 7.

Table 6. The number of major urban road links projected to exceed 8.59 ppm and 10 ppm 1998 to 2004

Table 7. The road links with the highest estimated roadside maximum 8-hour CO concentrations in 2002 (extreme meteorology, ppm)

5.2 Projections for Marylebone Road based on the mapping method

</>Figure 15 shows projections of the mapped maximum 8-hour roadside CO concentrations at Marylebone Road for both typical and extreme meteorology for all years from 1990 to 2010. The values for 1998 are 8.9 and 10.5 ppm respectively, which are higher than the individual site analysis result for this site of 6.5 ppm (Table 2). This is because the mapping method is designed to provide an estimate of the maximum possible concentration in each year. The individual site analysis for this site was derived from 1998 data and 1998 was a year of relatively low concentrations. The highest concentrations listed in Table 4 are higher than those estimated for Marylebone Road because there are several road links with higher traffic flows than Marylebone Road.

Measured maximum 8-hour CO concentrations at both background and roadside sites shown on Figure 15 are the maximum values from the whole of the UK networks for each year. The only roadside site in the years 1990 to 1995 was Cromwell Road and in many years higher concentrations were recorded at one or more of the background sites. Background concentrations were generally lower than our predicted maximum concentration for typical meteorology at roadside sites and always lower than our predictions for extreme meteorology. Data for more roadside sites are available from 1996 onwards and the measured maximum roadside concentration was higher than the maximum background in the years 1996, 1997 and 1998. Our predictions for extreme meteorology appear at Marylebone Road to give a safety margin of between 10 to 50% over the measured maximum 8-hour CO concentrations at roadside and background monitoring sites.

The 'worst case' projections that were calculated for the review of the NAQS are also shown on Figure 15. These projections were calculated by adding the maximum 8-hour roadside enhancement of concentrations at Marylebone Road to the maximum running 8-hour mean in Central London. It is clear that this method of adding the maxima gives much higher concentrations than the mapping methods used here and that the mapping method provides more realistic representation of concentrations.

Chapter 4         Chapter 6

Report and site prepared by the National Environmental Technology Centre, part of AEA Technology, on behalf of the UK Department of the Environment, Transport and the Regions